The x Digital Subscriber Line (xDSL) technology may be divided into rate symmetric type and rate asymmetric type according to whether the uplink (from a user to the switching office) rate is the same as the downlink (from the switching office to a user) rate. The xDSL of asymmetric type includes Asymmetric DSL (ADSL) and Very high bit rate DSL (VDSL), which is applicable for the Internet service in which the downlink data quantity is large since its downlink rate is very high.
The ADSL technology has developed from the first generation ADSL to the current second generation ADSL2, ADSL2+ and VDSL2, the frequency band of the ADSL is gradually increased and the corresponding bandwidth is gradually increased too. And the ADSL and ADSL2 use the spectrum less than 1.1 MHz as the downlink bandwidth to provide a maximal downlink rate of 8 Mbps; ADSL2+ extends the downlink bandwidth to 2.2 MHz to provide a maximal downlink rate of 24 Mbps; VDSL2 even uses 30 MHz spectrum to provide a maximal uplink/downlink rate of 100 Mbps.
The prior xDSL access device is generally located at a Central Office (CO); with the increase of bandwidth requirement, the xDSL is required to be closer to the user. Therefore, the application pattern of the xDSL access device located at the Remote Module (RT) is gradually increased, and in particular, VDSL2 is mainly located at the location close to the user such as districts, buildings and roadsides, which leads to some spectrum compatibility problems, for example, a Digital Subscriber 1 (DS1) signal amplitude of the xDSL access device located at the CO is very small due to attenuation. The crosstalk between the downlink DS1 signal of the xDSL access device (ADSL2+ device of the remote Digital Subscriber Line Access Multiplexer (DSLAM) module 100 as shown in FIG. 1) close to the user terminal and the downlink DS2 signal of the xDSL access device located at the CO will influence the Signal to Noise Ratio (SNR) of DS1 signal, and result in the performance reduction. Therefore, it is necessary to use some measures to reduce the influence caused by the crosstalk. Due to the above problem, many operators constitute theirs own spectrum application management criterions to stipulate the spectrum planning in various application cases, which avoids the performance descent caused by the mutual interference between various devices.
One prior art associated with the present invention is the solution described in an European patent, patent number EP1370057, in the European patent the fixed spectrum settings is used to avoid the crosstalk by closing some carriers of relevant frequency band (tone disabling) according to the location and requirement of the remote module. As shown in FIG. 2, the overlapping portion (as the dot line shown in FIG. 2) between the DS signal of the frequency band from 1 MHz to 2 MHz in the remote ADSL2 and the DS signal of the downlink frequency band in the CO ADSL is closed to reduce the downlink interference in the CO ADSL. The crosstalk between the downlink signal of the remote module and the downlink signal of the CO module may be avoided since two spectrums are not overlapped any more.
As can be seen from the solution of the above prior art, the spectrum compatibility requirement may be met in the prior art, but the remote DSLAM only uses the carrier of the frequency band more than 1.1 MHz; the carrier performance rapidly descends along with the increase of line distance since the attenuating degree of the carrier within the frequency band more than 1.1 MHz is greater than that within the frequency band less than 1.1 MHz along with the increase of the line distance, so that the performance index of the remote xDSL access device is severely restricted. Practically, the CO signal may be transmitted by means of the relatively low power spectrum density since the line has certain crosstalk rejection capability (the parameter for quantificationally describing the crosstalk rejection capability of the line is referred to as crosstalk rejection ratio), which does not influence the CO service greatly.
In addition, the fixed spectrum settings is used in the prior art, while the spectrum in the line is dynamically changed practically. Therefore, the system using the fixed spectrum settings is unable to adapt for the spectrum change in the line dynamically, which makes the flexibility worse and the spectrum utilization rate lower.